1. Field of the Invention
This invention relates to fiber optic systems for early detection and location of mechanical, thermal, and other disturbances. In particular, it relates to such systems, which are used for structural monitoring of large structures and for detecting and locating fluid presence.
2. Description of Prior Art
Fiber optic sensors become attractive innovation with applications in many areas of industry and engineering. Optical fiber, due to its properties, can play roles of both sensitive element and signal delivery channel. Especially distributed fiber optics sensors are suitable for monitoring of large pieces of equipment such as planes, ships, factory equipment, pipelines, and construction structures like buildings, bridges, etc.
In recent years, many fiber optic sensing systems were developed using in-fiber Bragg gratings. Bragg gratings have narrow reflection spectral bands whose position within the optical spectrum depends on certain conditions, like temperature and axial strain. With additional components sensitive to other physical and chemical factors and mechanically connected to optical fibers with embedded Bragg gratings, such sensors become useful for measurement and detection of these factors as well.
Sensor demodulation, the recovery of information on monitored condition or factor from the optical reflection spectra of the fiber, in case of Bragg grating sensors, is performed by wavelength scanning using tunable lasers or spectrum analyzers. Also, demodulation schemes using single and double wavelength methods have been suggested where the light source emits light at one or two wavelengths within the reflection spectrum of the grating and the reflected light intensity changes with a shift of Bragg grating spectrum.
Multiplexing of fiber optic sensors enable the use of single optical fiber, light source and detecting instrumentation for distributed sensing at many points or segments along the fiber simultaneously Bragg grating based sensors can be multiplexed using wavelength division, if each Bragg grating occupies its own spectral range. Also, if spectra of Bragg gratings occupy the same wavelength range and each grating is being weakly reflective, the multiplexing can be performed using whether time domain or frequency domain reflectometry techniques.
However, it is well recognized that complete sensing systems based on in-fiber weakly reflective Bragg gratings which includes optical fiber sensing network, signal demodulation and demultiplexing subsystems are still quite expensive. The combination of light sources and high-sensitive detectors capable of multi-band high-resolution spectral analysis or/and complex time or frequency domain reflectometry does not yet provide cost effective solution for widely acceptable applications of distributed fiber optic sensors. Thus, the development of cost effective solution for fiber optic distributed sensors may be regarded as an important objective.
Failure in physical structures begins, as a rule, in a small fashion and increases with time. Early detection of failure can prevent costly or even catastrophic results of a completed failure. A cost-effective system, which could detect the onset of failure, would be of great benefits. Examples of structures that could benefit from strain level inspection, flaw and the onset of failure detection include bridges and buildings, electrical power transmission lines, long pipelines, etc. Other areas that could benefit from such inspection include structures where visual human inspection is difficult or impossible, such as underground storage tanks and underwater pipes, etc.
Fiber optic sensing techniques are very promising for such applications where a distributed sensing or multiplexing of a multiplicity of discrete sensors is required. There are a number of approaches that use disturbances in an optical beam traveling through an optical fiber as a method of detecting mechanical, thermal, and other disturbances in a structure that is to be inspected. Typically, an optical fiber is appropriately placed, affixed or embedded into a structure in such a way that disturbances in the structure could effect the optical fiber and be transferred into the fiber disturbance. The behavior of the optical beam is then monitored within the optical fiber. Books with comprehensive reviews on fiber optic sensing techniques have been published recently (see, for example, J. Dakin, B. Culshaw, editors. Optical fiber sensors, application analysis and future trends. Vol 3, 4. Artech House, London 1997; K. T. V. Gratan and B. T. Meggit, Editors. Optical Fiber Sensor Technology, Vol. 2, Chapman & Hall, London, 1998), which discussed many approaches suggested or under development for these purposes.
Fiber optic sensing systems based on different principles, such as an interferometric, the microbending induces losses, nonlinear effects in optical fibers, in-fiber Bragg gratings, etc., have been proposed and developed.
Most of the sensors utilizing the interferometric principle measure changes in the total length of the fiber. In particular, fiber optic Sagnac interferometer and the Mach-Zehnder interferometers have been proposed for varying disturbance detection (U.S. Pat. Nos. 5,636,021 and 5,046,848 to Udd, U.S. Pat. No. 5,355,208 to Crawford , et al.). However, this approach does not allow a simple locating the multiple disturbances. In particular, there are no convenient and economical means for determining the location of static or slowly varying disturbance along the extensive sensing loop.
Another technique utilizes the intensity modulation of an optical beam in the fiber that occurs from changing “microbends” in the fiber (U.S. Pat. No. 4,891,511 to Reed S., U.S. Pat. No. 4,477,725 to Asawa et al., U.S. Pat. No. 4,727,254 to Wlodarchyk, et al.). Although the microbend sensor could provide a simple solution for many applications and has capability for locating a disturbance by using OTDR technique, this method suffers from a high level of losses of the signal pulse energy in the sensing optical cable and very small energy of scattered back signal. That limits significantly a sensing length of the detector system and reduces speed of measurements because the large number of averages needed to achieve acceptable resolution.
Different distributed fiber optical sensors using nonlinear effects in fibers, such as the Kerr effect in polarization maintaining fibers have been proposed (U.S. Pat. No. 5,627,637 to Kapteyn), the Raman and Brillouln light scattering effects in multi-mode and single-mode fibers (U.S. Pat. Nos. 5,949,533 , 5,767,956 , 5,272,334). Such sensors provide means for distributed temperature or/and strain measuring, however their functioning is based on propagating in fiber the powerful pumping light pulses and complicated signal detection and processing. The main drawback of these sensors is their very high complexity and cost.
The FBG sensors are particularly attractive for quasi-distributed sensing because many Bragg gratings can be written into the length of fiber and be multiplexed. It is known that Bragg gratings written in optical fibers may be used to detect perturbations, such as strain or temperature, at the location of the gratings, as is described in U.S. Pat. No. 4,806,012 and 4,761,073, both to Meltz et al. However, such Bragg grating sensors require a high-resolution spectrometer to determine the sensor response, to determine the wavelength shift for each of the gratings, and to multiplex from one grating to the next. Such a spectrometer-based system is costly, delicate, and does not permit to multiplex many grating sensors. One of the main obstacles in implementing a practical system has been the development of multiplexing instrumentation capable of resolving a large number of concurrent signals at the relatively low power levels that are desirable. Moreover, the system should be tolerant of relatively long fiber lengths and permit patterns of distribution of sensors that accommodate if necessary some variations. A distributed Bragg grating-based sensor has been proposed by A. Kersey (U.S. Pat. No. 5,757,487) which combines wavelength- and time-domain multiplexing in order to increase a number of sensing points. However, this sensor is still complicated and requires the use of a multi-wavelength pulsed light source and a special multi-wavelength multiplexer.
Thus, the development of cost effective solution for fiber-optic distributed detection and location of disturbances may be regarded as an important objective.